The present invention relates to sampling methods and apparatus for low power applications.
In medical applications, it is often required to monitor patient physiological parameters at all times, even when that patient cannot be connected to a patient monitor. This requires that a remote monitor, be carried by the patient. The monitor, in turn, is connected to electrodes placed at appropriate locations on the body, for example, electrocardiogram electrodes. The analog signals generated by these electrodes are received by the remote monitor. These signal are generally converted to digital samples which may either be stored in the monitor or transmitted via a telemetry link to a central monitor. In either event, the monitor must necessarily be battery powered, and consequently must be made to minimize power consumption. Such a monitor may also be subject to induced electrical noise.
Digital signal processing systems are well known, including those which operate on analog input signals. To process analog input signals, digital samples are taken of the analog signal by a sampler. The samples are then processed by the digital signal processing system. In most such systems, the sampling rate is fixed at a predetermined rate (see U.S. Pat. No. 5,229,668, issued Jul. 20, 1993 to Hughes, Jr. et al.; and U.S. Pat. No. 5,797,399, issued Aug. 25, 1998 to Morris et al.). Such systems also include sample rate converters, which receive samples at a first fixed rate, and produce corresponding samples at a second fixed rate (see U.S. Pat. No. 5,907,295, issued May 25, 1999 to Lin; U.S. Pat. No. 5,936,438, issued Aug. 10, 1999 to Whikehart et al.; and U.S. Pat. No. 5,982,305, issued Nov. 9, 1999 to Taylor). In other such systems, the sample rate is settable, and may be varied from one system implementation to another, or from processing one input signal to another, but once preset, the sample rate remains fixed at the preset rate (see U.S. Pat. No. 5,375,067, issued Dec. 20, 1994 to Berchin; U.S. Pat. No. 5,400,371, issued Mar. 21, 1995 to Natarajan; and U.S. Pat. No. 5,645,068, issued Jul. 8, 1997 to Mezack et al.).
Other systems can have their sample rate varied during use. Some such systems are used where a single analog input signal must be processed by different processing circuits which operate at respectively different sample rates. Systems of this type can vary the sample rate dynamically depending on the sample rate currently required by the processing circuitry (see U.S. Pat. No. 5,625,359, issued Apr. 29, 1997 to Wilson et al.).
In another system, samples are not taken uniformly, but at locations dependent on the input signal. In U.S. Pat. No. 3,023,277, issued Feb. 27, 1962 to Mathews, an input signal is sampled at positive and negative peak values of the input signal, whenever they occur, and samples representing those peak values are then further processed.
In yet another system, the sample rate is dynamically varied at times when more detail about the input signal is desired. For example, in systems adapted for remote operations under battery power, there is limited power, and usually limited storage for samples. In such systems, the sample rate is generally kept low. Only when some event of interest occurs, and a more detailed record of the input signal is desired, is the sample rate increased. Because it is well known that sampling at a higher rate takes more power than sampling at a lower rate, and because sampling at a higher rate is limited to only those times when it is needed, this technique conserves power. In addition, because fewer samples are taken at times when nothing of interest is occurring, the storage capacity, and thus the number of memory circuits required to store the samples, is reduced, further reducing the power required (see U.S. Pat. No. 4,827,259, issued May 2, 1989 to Murphy et al.; and U.S. Pat. No. 5,323,309, issued Jun. 21, 1994 to Taylor et al.)
It is also a well-known problem for input signals to include not only a signal component but also a noise component, both of which are converted to digital form when an analog signal is digitized by the sampler. Such a noise component usually has higher frequency content than the signal component. To remove noise at a higher frequency than the signal component, prior art systems fixed the sample rate so that it satisfied the Nyquist criterion for the highest frequency in, or expected to be in, the noise component and then filtered the resulting sample sequence to attenuate the noise component. However, as described above, increasing the sample rate of the sampler increases the power consumption of the digital processing system, and the storage requirements for the samples taken.
It is desirable to sample an analog input signal, including a signal component and possibly also a noise component, for processing in a digital signal processing system, in a manner which minimizes the noise present in the digital samples, while simultaneously minimizing the power consumption of the data acquisition system.
The inventor realized that in some conditions, the input signal does not have a noise component. The inventor further realized that under these conditions, it is not necessary for the sampling rate to satisfy the Nyquist criterion for the (non-existent) noise component. Instead the sampling rate may be decreased to the point where it satisfies the Nyquist criterion for the signal component alone.
In accordance with principles of the present invention, a sampling system includes an input terminal for receiving a data signal having a signal component and possibly a noise component. A sampler samples the data signal at a sample rate set in responsive to a control signal. A noise detector detects the presence of a noise component, and if a noise component is detected, generates the control signal conditioning the sampler to sample the data signal at a first sample rate satisfying the Nyquist criterion for the data signal including the noise component, and otherwise generating the control signal conditioning the sampler to sample the data signal at a second data rate satisfying the Nyquist criterion for the data signal including only the signal component.
A sampling system according to the above invention is optimized to the signal actually being received. If the input signal contains only the signal component, with no noise component, then the sampling rate is decreased, thereby minimizing the power required. Only when a noise component is detected, the sampling rate is increased so that the noise component may be filtered out. In this manner, power is conserved to the extent possible.